Mesa Fixed bias...

Hey guys,

Does anyone know about "trim pot"?

I recently went to Eurotubes.com and read this from the guys website:

"Mesa amps are fixed bias and are set pretty cold. The Dual and Triple rectifiers are set very cold and can benefit greatly from having a trim pot installed and the bias adjusted properly"

I have an F-50, and would like to know if I should do this before I get new tubes?

How much would it cost to do all this? Biasing? trim pot?

I will be recording at the studio in 3 weeks and wanted to get the best sound possible. I bought the amp brand new from GC back in Oct. 2004

I appreciate all comments and opinions on this subject.

Thanks.

biasing is way overated bro

pau_leader said:

I recently went to Eurotubes.com and read this from the guys website:

"Mesa amps are fixed bias and are set pretty cold. The Dual and Triple rectifiers are set very cold and can benefit greatly from having a trim pot installed and the bias adjusted properly"

Click to expand...

This is matter of preference. Some people likes it set hot some like it set cold.

When I was at amp repair shop, I asked about installing an adjustable bias, the technician responded that there is some trade-off installing one and I was better off keeping the set bias and just purchasing the tubes recommended in the M/B manual.

Now for some of you this might not set well since one of you stated you receive some bad batch of M/B tubes. But for me, I prefer set bias and get the recommend tubes. I don't like opening a can of worms with swapping tubes, adjusting bias etc ...

Hope others chime in about adjustable bias, (note: I'm not here to debate about "fix bias" -vs- "adjustable bias" I just gave my preference).

RR said:

pau_leader said:

I recently went to Eurotubes.com and read this from the guys website:

"Mesa amps are fixed bias and are set pretty cold. The Dual and Triple rectifiers are set very cold and can benefit greatly from having a trim pot installed and the bias adjusted properly"

Click to expand...

This is matter of preference. Some people likes it set hot some like it set cold.

When I was at amp repair shop, I asked about installing an adjustable bias, the technician responded that there is some trade-off installing one and I was better off keeping the set bias and just purchasing the tubes recommended in the M/B manual.

Now for some of you this might not set well since one of you stated you receive some bad batch of M/B tubes. But for me, I prefer set bias and get the recommend tubes. I don't like opening a can of worms with swapping tubes, adjusting bias etc ...

Hope others chime in about adjustable bias, (note: I'm not here to debate about "fix bias" -vs- "adjustable bias" I just gave my preference).

Click to expand...

I agree.

If you already like the sound of your amp I would just keep the fixed bias. If you like to tweak things you could get a trim pot to adjust your bias.

You could make the bias setting hotter and maybe get a better sound but you would also probably shorten the life of your tubes. Maybe not by a large degree but there are always tradeoffs.

By the way this is pure personal opinion. Not based on any fact.

I think the Mesa Rectos sound just a touch better when they are biased up hotter. The overall dynamics of the amp improve a good bit.

If you're listening close enough, you can hear the difference between a Recto that's biased up to about 40 to 50% plate dissipation and a Recto that biased up between 60% and 75% plate dissipation.

The only trade-off is that the Plate Voltage and Cathode Current (Bias Current) will change every time you swap to a different mode with a Dual or Triple Rectifier. Bold/Silicon Diodes has the highest plate voltage and will have the highest Cathode Current (Bias). Spongy/Tube will have the lowest plate voltage and the lowest Cathode Current.

I'll use some of Mesa's terminology here: In other words, the Bias will not "Track" along with each mode.

getting a bias pot installed will also void the hell out of your warranty

Yeah, thanks for all the info. I love the sound I get from the amp.
Ill just get the tubes. You guys are cool.

Dale B. said:

...The only trade-off is that the Plate Voltage and Cathode Current (Bias Current) will change every time you swap to a different mode with a Dual or Triple Rectifier. Bold/Silicon Diodes has the highest plate voltage and will have the highest Cathode Current (Bias). Spongy/Tube will have the lowest plate voltage and the lowest Cathode Current.

I'll use some of Mesa's terminology here: In other words, the Bias will not "Track" along with each mode.

Click to expand...

Dale, does this mean that there will be crossover distortion everytime a mode is changed? Would a workaround be setting the bias for the "hottest" mode (Bold/Diode/Modern) and just accepting that it will run a little cooler on the lower settings (i.e., Spongy/Tube/Vintage)?

...or am I completely spun around here?

Erik said:

Dale B. said:

...The only trade-off is that the Plate Voltage and Cathode Current (Bias Current) will change every time you swap to a different mode with a Dual or Triple Rectifier. Bold/Silicon Diodes has the highest plate voltage and will have the highest Cathode Current (Bias). Spongy/Tube will have the lowest plate voltage and the lowest Cathode Current.

I'll use some of Mesa's terminology here: In other words, the Bias will not "Track" along with each mode.

Click to expand...

Dale, does this mean that there will be crossover distortion everytime a mode is changed? Would a workaround be setting the bias for the "hottest" mode (Bold/Diode/Modern) and just accepting that it will run a little cooler on the lower settings (i.e., Spongy/Tube/Vintage)?

...or am I completely spun around here?

Click to expand...

Yeah, you could do that and that's what most folks do who have had their Retcos modified with Bias Pots. They just have the bias set to whatever mode they're using all of the time and forget about it. It's just if you set it for anyone of the tube recto modes or anything that's in the Spongy mode and you decide to go up from there, you're asking for trouble.

As for me, I think the amp sounds fine enough as it is and it's one less thing for you to have to worry about. The only thing that sucks is the price of the tubes Mesa sells. The other thing that sucks is that they don't sell any tubes that I like other than the STR-442, which is the SED EL34 (the old Svetlana). I just buy my tubes from other places.

Dale

Thanks Dale...that's sort of what I thought. I just realized that I need to re-tube my Road King and I need to explain that to my tech. (If he doesn't already know.)

Erik said:

Thanks Dale...that's sort of what I thought. I just realized that I need to re-tube my Road King and I need to explain that to my tech. (If he doesn't already know.)

Click to expand...

Has your Road King been modified for a bias pot?

Dale B. said:

Erik said:

Thanks Dale...that's sort of what I thought. I just realized that I need to re-tube my Road King and I need to explain that to my tech. (If he doesn't already know.)

Click to expand...

Has your Road King been modified for a bias pot?

Click to expand...

Yes...I had it done by Voodoo Amps. I'm currently using Ruby EL34 BSTR, and KT66's in place of the 6L6's. In the V1,2,and 3, I'm using the 9th generation Chinese 12ax7's...in the PI I've got a 12AT7 and a EH12ax7 in V5. That pretty much nails the tonal spectrum I was after...however, I'm looking to retube the KT66's with an SED quad.

Man, I wished you lived nearby...we could have some fun with this thing.